To be cost efficient, terrestrial electrical generators under 1,000 HP use reciprocating engines as their power source. Reciprocating engines use up a lot of closed space which has to be adapted to tolerate heavy weights and medium- and low-frequency vibrations, high maintenance costs and a narrow range of fuels. Furthermore, these engines take a long time to warm up and get into condition for connecting to the power lines, which hinders their availability to swiftly respond to demand or else causes high maintenance costs when it is stopped for servicing to be carried out while still hot to reduce the outage time.
Light airplanes and helicopters needing power plants under 500 HP use internal-combustion engines. Compared to turbo-engines, these engines are heavier per unit of delivered power, highly complex because of the large quantity of moving parts they contain and require periodic specialized inspections.
Light one-to-four seating helicopters are particularly penalized because there is no alternative for them other than combustion engines. Therefore, their capabilities are severely restricted by the need to carry a heavy power plant, a significant weight compared to one or more passengers. Stresses and vibrations transmitted to the whole helicopter and to the passengers or the use of reciprocating engines further significant deter use of these helicopters.
The high market prices of both light airplanes and helicopters have made room for the use of turbines instead of combustion engines. A difference compared to combustion engines, wherein the different strokes of the ignition cycle are carried out reciprocating in cylinders (in pulses), is that turbines carry out their ignition-compression process continuously. Turbines comprise a compression stage for producing pressurized air, a combustion chamber into which the pressurized air is admitted together with fuel and an expansion stage producing power on a rotary shaft by means of a turbine integrated to the compressor. Part of the power generated by the turbine is used for driving the compressor and auxiliary systems (e.g. alternators, pumps, etc.), the balance is available as net power.
A favorable feature of turbines is their ability to generate a high density of mechanical energy per volume-unit compared to combustion engines of like power. On the other hand, an unfavorable feature is that turbo-engines lack massive oscillating or eccentric mechanisms, hence dynamic high-amplitude and low-frequency stresses are transmitted to the structure thereof.
A further competitive advantage of turbo-engines vis-a-vis combustion engines is the former's greater flexibility in the election of fuel. Combustion engines may only use aircraft petrol which is very volatile and explosive, leading therefore to safety concerns. On the other hand, turbo-engines may be fuelled with aircraft kerosene (JP1), which is much less volatile and explosive, natural gas, diesel-oil or practically any kind of fossil or synthetic fuel producing less emission of polluting gases compared to combustion engines of like power. The possibility of using cheap fuel makes turbo-engines moreover more attractive for terrestrial or stationary uses for generation of electricity or delivering work to a shaft.
In addition, the balance of heat which is emitted as well as the high-temperature exhaust gases may be advantageously used, either for supplying a heating system or for recovery in a secondary cycle, leading to improved efficiency of the turbo-engine cycle. Furthermore, turbo-engines are less sensitive than combustion engines to loss of atmospheric pressure and low temperatures, as occurring at high flight altitudes. Aircraft turbines are firmly established in the high power segment, say above 600 HP.
An important technical obstacle that must be overcome when designing a low-power turbo-engine turning at high speeds (over 25,000 RPM, typically up to 60,000 or 70,000 RPM) is that its applications generally require a substantially lower number of revolutions per minute (RPM), meaning that an associated gear-box is necessary but which otherwise would be undesirable in that it adds weight, mechanical complexity and auxiliary systems.